1. Field of the Invention
The present invention relates to a focusing servo method for moving an objective lens to a focus position based on focus error signal and a device therefore.
2. Description of Related Art
Conventionally, it is known to provide a focusing servo device to an optical displacement gauge for detecting focus by reflected light obtained by irradiating laser beam. Examples of such focusing servo device are disclosed in Japanese Laid-Open Publications Hei 7-103710, Hei 7-294800 and Hei 9-68407.
The focusing servo device disclosed in the Japanese Laid-Open Publications Hei 7-103710, Hei 7-294800 and Hei 9-68407 has an objective lens moved toward and away from measurement surface and detects zero-cross point of S-curve signal generated by converting reflected light into an electric signal by the movement of the objective lens, so-called focus error signal, as a focus point. The objective lens is moved to the detected zero-cross point position by driving an actuator by outputting signal from CPU (Central Processing Unit) to obtain precise focus.
In the above-described conventional focusing servo device, the time from detection of the zero-cross point to movement of the objective lens to the focus position as the zero-cross point position depends on response characteristics of command transmission of control process of control system, in which the waveform of the focus error signal generated from the reflected light satisfies the condition of focusing area and the objective lens is moved under control of CPU. In the control process, mechanical component such as actuator and movement mechanism for moving objective lens by driving the actuator are provided. Accordingly, in order to prevent delayed or excessive movement of the objective lens relative to command signal in transmitting the command of the control system, feedback control is conducted while checking the current position of the objective lens.
However, since the conventional focusing servo device conducts feedback control while checking the current position of the objective lens, it takes much time before completing focusing on account of delay in command transmission, i.e. the time required for transmitting the command, in the control system, which hinders improvement in work efficiency.
On the other hand, knife-edge method and pinhole method are used for detecting the convergent reflected light for detecting the focus error signal in the above focusing servo device.
In the knife-edge method, the obtained focus error signal can be detected when the objective lens is shifted relative to actual focus position as shown in FIG. 17. Accordingly, the focus position can be recognized based on the focus error signal even if the objective lens is shifted from the focus position to a degree.
On the other band, the pinhole method has greater peak value of curve waveform of the focus signal and superior S/N ratio, i.e. small noise. Accordingly, the quality of focus error signal is better than the knife-edge method and focus can be precisely adjusted.
However, since the width of waveform of the focus error signal is narrow and the focus error signal cannot be detected when the position of the objective lens is shifted from the focus position to a degree, the measurement surface cannot be recognized and focusing process is not conducted.
Accordingly, in order to use the pinhole method, additional step such as control by an operator for detecting focus error signal by operating a switch etc. is required, so that process automation cannot be achieved and focusing process can be complicated.